It is well known that manufacturing costs directly affect a products profit margin. This is especially true where extremely high volumes are concerned. In the telecommunications industry for example, line interface cards as exemplified in U.S. Pat. No. 5,333,192, interface telephone sets to end offices and are manufactured by the millions. Even a cost savings of pennies in the manufacturing of a single line card can add up to a staggering overall savings where a manufacturer is manufacturing in excess of ten million of them annually.
On the manufacturing assembly line savings can be realized through a reduction in the total number of components that have to be mounted onto a line card printed circuit board. In an attempt to realize savings in this manner manufacturers have for many years looked to optimizing circuit design in the hopes of reducing component count.
Currently some manufacturers of line card interface circuits utilize two different component mounting technologies in every line card manufactured. Through hole mounting as well as more recent surface mount manufacturing techniques are often used at different stages of the manufacture. The added expense of surface mount capability is staggering as a result of equipment cost and the additional floor space required to house it. This problem is compounded in today's global market environment where customers essentially demand manufacturing facilities in their respective countries as a condition of sale. To respond, manufacturers interested in selling to global markets very often have to duplicate many times over, manufacturing facilities which include the two mounting technologies.
Earlier attempts by industry and even by the applicant to construct a complete line card circuit on ceramic substrate (e.g. Nortel.RTM. product DMS-1 Urban Line cards; 1983) have largely been unsuccessful. Much of the reason for the lack of success has been attributed to thermal problems as ceramic substrate is a better conductor of heat than more conventional fibreglass substrates resulting in heat generated from hot components interacting with other adjacent components in a compromising manner. By design, battery feed resistors dissipate significant heat which in turn affects temperature sensitive integrated circuits providing coding and decoding (Codec) functionality. Other heat generating integrated circuits such as Field Effect Transistors (FETs) or custom silicon often used in current limiting circuitry, further compound the heat problem. Two major thermal concerns which directly affect reliability include component solder joint fatigue and integrated circuits operating at compromising junction temperatures. Integrated circuits very often used in line card circuits include codec functionality and operating at increased temperatures severely affects transmission characteristics such as absolute channel noise, quantization distortion and absolute gain. The smaller the ceramic substrate used the more significant the thermal problem encountered. Increasing the ceramic substrate to a size where thermal problems are virtually non existent is not the solution as no savings are realized due to the high cost of the ceramic.
As large ceramic substrate structures are extremely expensive and in view of many known problems affecting reliability resulting from their inherent thermal properties, manufacturers today have limited there use to providing small printed feed resistor structures which occasionally include non heat sensitive passive components such as capacitors and other non printed resistors. Service providers for years have benefited from using ceramic structures in this manner as they provide known advantages such as providing very accurately trimable resistors and a desirable known failure mode in extreme conditions such as in the event of lightening strikes.